Abstract
Long-term periodicities in the solar irradiance are often observed with periods proportional to the solar rotational period of 27 days. These periods are linked either to some internal mechanism in the Sun or said to be higher harmonics of the rotation without further discussion of their origin. In this article, the origin of the peaks in periodicities seen in the solar extreme ultraviolet (EUV) and ultraviolet (UV) irradiance around the 7, 9, and 14 days periods is discussed. Maps of the active regions and coronal holes are produced from six images per day using the Spatial Possibilistic Clustering Algorithm (SPoCA), a segmentation algorithm. Spectral irradiance at coronal, transition-region/chromospheric, and photospheric levels are extracted for each feature as well as for the full disk by applying the maps to full-disk images (at 19.3, 30.4, and 170 nm sampling in the corona/hot flare plasma, the chromosphere/transition region, and the photosphere, respectively) from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) from January 2011 to December 2018. The peaks in periodicities at 7, 9, and 14 days as well as the solar rotation around 27 days can be seen in almost all of the solar irradiance time series. The segmentation also provided time series of the active regions and coronal holes visible area (i.e. in the area observed in the AIA images, not corrected for the line-of-sight effect with respect to the solar surface), which also show similar peaks in periodicities, indicating that the periodicities are due to the change in area of the features on the solar disk rather than to their absolute irradiance. A simple model was created to reproduce the power spectral density of the area covered by active regions also showing the same peaks in periodicities. Segmentation of solar images allows us to determine that the peaks in periodicities seen in solar EUV/UV irradiance from a few days to a month are due to the change in area of the solar features, in particular, active regions, as they are the main contributors to the total full-disk irradiance variability. The higher harmonics of the solar rotation are caused by the clipping of the area signal as the regions rotate behind the solar limb.
Highlights
Investigation of periodicities in solar observational data from both ground-based and satellite space missions has long been of high interest to understand the solar variability
We have published several papers on the segmentation of coronal and photospheric features and their time series using spatially resolved full-disk images of the Sun observed from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory spacecraft (SDO), as well as the Sun Watcher using APS and Image Processing (SWAP) and the Large Yield Radiometer (LYRA), both on board the PROBA2 spacecraft to understand the extreme ultraviolet (EUV) and ultraviolet (UV) irradiance variability (Kumara et al, 2012, 2014; Zender et al, 2017)
It is important to point out that the AIA images fed to the Spatial Possibilistic Clustering Algorithm (SPoCA) algorithm were not corrected for the degradation, which might have slightly affected the segmentation towards the end of the 2011 – 2018 period
Summary
Investigation of periodicities in solar observational data from both ground-based and satellite space missions has long been of high interest to understand the solar variability. The previous results were based on data sets that are not long enough to get any statistically significant periods, and these long periods may originate from the computational techniques used, such as the way of data detrending and smoothing The existence of these periods depends strongly on the solar activity and the time interval that has been investigated. We have published several papers on the segmentation of coronal and photospheric features and their time series using spatially resolved full-disk images of the Sun observed from the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory spacecraft (SDO), as well as the Sun Watcher using APS and Image Processing (SWAP) and the Large Yield Radiometer (LYRA), both on board the PROBA2 spacecraft to understand the extreme ultraviolet (EUV) and ultraviolet (UV) irradiance variability (Kumara et al, 2012, 2014; Zender et al, 2017).
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